Transcript Factors Causing Coral Bleaching and the Symbiotic

Factors Causing Coral Bleaching
and the Symbiotic Relationship
with Zooxanthellae
By Veronica Rodriguez
Ecol 475
Coral Bleaching
Coral Bleaching is a stress condition in coral reefs
that involves the breakdown of zooxanthellae.
Zooxanthellae
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Zooxanthellae are dinoflagellate algae of the
genus Symbiodinium, and live in coral tissue.
They provide nutrients such as sugars and
oxygen that are essential for the production of
calcium carbonate (coral reef skeleton).
Provide coral with beautiful coloration (without
them corals are clear or white; phenomenon
known as coral bleaching).
More than 50% of the corals’ nutrients are
derived from photosynthetic products produced
by zooxanthellae.
Coral Reefs
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Coral Reefs are most diverse and productive
communities on Earth.
They are referred to as the “Tropical Rainforest of
the Ocean”
Found in warm, clear, shallow waters.
Provide zooxanthellae with a protected
environment, steady supply of carbon dioxide,
and nitrogen.
Factors Affecting Coral Bleaching
Temperature: Fluctuation of low and high sea temperature
accompanying intense upwelling.
 Winter: Change from -3 C to -5 C for 5 to 10 days
 Summer: Change from 1 to 2 C for 5 C to 10 days
(more common)
Salinity: Zooxanthellae are sensitive to low salinity due to
precipitation and runoff. Therefore, they tend to live near
shallow, clear waters (no deeper than 100m) with plenty of
sunlight.
UV radiation: High levels of photosynthesis leads to high
amounts of nitric oxide production by zooxanthellae.
More Factors
Natural Phenomenon: Violent storms, flooding, ENSO (El
Niño Southern Oscillation), predatory outbreaks, and
tsunami are devastating to coral reefs.
Anthropogenic Phenomenon: Overexploitation,
overfishing, increased sedimentation, and nutrient
overloading.
Map of coral bleaching over the past 15 years, and major coral
bleaching events (yellow dots).
www.iper1.it/.../ index/cerca/reef
Research
Study done by Rosenberg and Ben-Haim (2002):
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The actual agent leading to bleaching of hard coral, Oculina
patagonia, is caused by an infectious bacteria called Vibrio
shiloi.
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Purpose of the experiment was to test the growth of
bacteria within two different sea water temperatures in an
aquaria environment.
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Results showed bacteria produced an extracellular toxin
that blocked photosynthesis within the zooxanthellae
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The toxin would bind to the algal membranes and prevent
the ammonia and lead to the destruction of pH gradient
across the thylakoid membrane.
Research 2
Study done by Rosenberg and Falkovitz, 2004:
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Similar study done with Oculina patagonica, hard coral and
Vibrio shiloi, bacteria.
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Results showed the inability of the bacteria to bleach the
coral at cold temperatures due to increased resistance of
the coral and in hot temperatures the bacteria was able to
produce.
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Laboratory experiment demonstrated high temperature
would cause bleaching by the bacteria.
• Temperature at 29 C there was rapid and complete bleaching
• At 25 C bleaching was slower and incomplete
• Below 20 C there was no bleaching, even with large amounts
of the bacteria exposed.
Conclusion
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Both studies showed the bacteria was unable to grow in
cold temperatures.
A theory was the zooxanthellae provided protection to the
coral from the intracellular bacteria.
• Based on an alternative explanation of the zooxanthellae
production and concentration of oxygen to the coral was toxic
to the bacteria.
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Results suggested the zooxanthellae are essential in
providing nutrients to coral, as well as, providing protection
to coral from the bacteria, V. shiloi.
Future studies are to be done on the symbiotic relationship
between zooxanthellae and coral reefs.
Human interaction should also be taken in consideration.
Global Warming
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Global warming is a major concern:
• If temperatures continue to increase to 1-2 C, for the
next 20 years there will be mass coral bleaching
worldwide.
• The high seawater temperature elevation will affect over
95% of the species living within the coral and lead
species to become extirpated or extinct.